1. Field of the Invention
The invention relates generally to earth-boring bits used to drill a borehole for the ultimate recovery of oil, gas, or minerals. More particularly, the invention relates to drill bits designed to shift the orientation of its axis in a predetermined direction as it drills. Still more particularly, the invention relates to a drill bit having inclination reducing or “dropping” tendencies.
2. Background of the Invention
An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by rotating the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. With weight applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone. The borehole thus created will have a diameter generally equal to the diameter or “gage” of the drill bit.
Many different types of drill bits and cutting structures for bits have been developed and found useful in drilling such boreholes. Two predominate types of rock bits are roller cone bits and fixed cutter (or rotary drag) bits. Many fixed cutter bit designs include a plurality of blades that project radially outward from the bit body and form flow channels there between. Typically, cutter elements are grouped and mounted on the several blades.
The cutter elements disposed on the several blades of a fixed cutter bit are typically formed of extremely hard materials and include a layer of polycrystalline diamond (“PD”) material. In the typical fixed cutter bit, each cutter element or assembly comprises an elongate and generally cylindrical support member which is received and secured in a pocket formed in the surface of one of the several blades. A cutter element typically has a hard cutting layer of polycrystalline diamond or other superabrasive material such as cubic boron nitride, thermally stable diamond, polycrystalline cubic boron nitride, or ultrahard tungsten carbide (meaning a tungsten carbide material having a wear-resistance that is greater than the wear-resistance of the material forming the substrate) as well as mixtures or combinations of these materials. The cutting layer is exposed on one end of its support member, which is typically formed of tungsten carbide. For convenience, as used herein, reference to “PD bit” or “PD cutter element” refers to a fixed cutter bit or cutter element employing a hard cutting layer of polycrystalline diamond or other superabrasive material such as cubic boron nitride, thermally stable diamond, polycrystalline cubic boron nitride, or ultrahard tungsten carbide.
While the bit is rotated, drilling fluid is pumped through the drill string and directed out of the drill bit. The fixed cutter bit typically includes nozzles or fixed ports spaced about the bit face that serve to inject drilling fluid into the flow passageways between the several blades. The flowing fluid performs several important functions. The fluid removes formation cuttings from the bit's cutting structure. Otherwise, accumulation of formation materials on the cutting structure may inhibit or prevent the penetration of the cutting structure into the formation. In addition, the fluid removes cut formation materials from the bottom of the borehole. Failure to remove formation materials from the bottom of the borehole may result in subsequent passes by the cutting structure to re-cut the same materials, thus reducing cutting rate and potentially increasing wear on the cutting surfaces. The drilling fluid and cuttings removed from the bit face and from the bottom of the borehole are forced and carried to the surface through the annulus that exists between the drill string and the borehole sidewall. Still further, the drilling fluid removes frictional heat from the cutter elements in order to prolong cutter element life. Thus, the number and placement of drilling fluid nozzles, and the resulting flow of drilling fluid, may significantly impact the performance of the drill bit.
Depending on the location and orientation of the target formation or pay zone, directional drilling (e.g., horizontal drilling) with the drill bit may be desired. In general, directional drilling involves deviation of the borehole from vertical (i.e., drilling a borehole in a direction other than substantially vertical), and is typically accomplished by drilling, for at least some period of time, in a direction not parallel with the bit axis. Directional drilling capabilities have improved as advancements in measurement while drilling (MWD) technologies have enabled drillers to better track the position and orientation of the wellbore. In addition, more extensive and more accurate information about the location of the target formation as a result of improved logging techniques has enhanced directional drilling capabilities. As directional drilling capabilities have improved, so have the expectations for drilling performance. For example, a driller today may target a relatively narrow, horizontal oil-bearing stratum, and may wish to maintain the borehole completely within the stratum. In some complex scenarios, highly specialized “design drilling” techniques with highly tortuous well paths having multiple directional changes of two or more bends lying in different planes may be employed.
One common method to control the drilling direction of a bit is to steer the bit using a downhole motor with a bent sub and/or housing. As shown in FIG. 1, a simplified version of a downhole steering system according to the prior art comprises a rig 1, a drill string 2 having a downhole motor 6 with a bent sub 4, and a conventional drill bit 8. Motor 6 and bent sub 4 form part of the bottomhole assembly (BHA) and are attached to the lower end of the drill string 2 adjacent the conventional drill bit 8. When not rotating, the bent sub 4 causes the bit face to be canted with respect to the tool axis. The downhole motor 6 is capable of rotating conventional drill bit 8 without the need to rotate the entire drill string 2. For example, downhole motor 6 may be a turbine, an electric motor, or a progressive cavity motor that converts drilling fluid pressure pumped down drill string 2 into rotational energy at drill bit 8. When downhole motor 6 is used with bent sub 6 without rotating drill string 2, drill bit 8 drills a borehole that is deviated in the direction of the bend or curve in the bent sub 6. On the contrary, when the drill string is also rotated, the borehole normally maintains a linear path or direction, even when a downhole motor is used, since the bent sub or housing rotates along with the drill string, and thus, no longer orients the drill bit in a specific direction. Consequently, a combination of a bent sub or housing and a downhole motor to rotate the drill bit without rotating the still string generally provide a more effective means for deviating a borehole.
When a well is deviated from vertical by several degrees and has a substantial inclination, such as greater than 30 degrees, the factors typically influencing drilling and steering may have a reduced impact. For instance, operational parameters such as weight on bit (WOB) and RPM typically have a large influence on the bit's ROP, as well as its ability to achieve and maintain the required well bore trajectory. However, as the inclination of the well increases towards horizontal, it becomes more difficult to apply weight on bit effectively since the borehole bottom is no longer aligned with the force of gravity—increasing bends in the drill string tend to reduce the amount of downward force applied to the string at the surface that is translated to WOB acting at the bit face. In some cases, the application of sufficient downward forces at the surface to a bent drill string may lead to buckling or deformation of the drill string. Consequently, directional drilling with a combination of a downhole motor and a bent sub may decrease the effective WOB, and thus, may reduce the achievable ROP.
In addition, as previously described, directional drilling with a downhole motor coupled with a bent sub is preferably performed without rotating the drill string in a process commonly referred to as “sliding.” However, in drilling operations where the drill string is not rotating, or is rotated very little, the rotational shear acting on the drilling fluid in the annulus between the drill string and borehole wall is decreased, as compared to a case where the entire drill string is rotating. Since drilling fluids tend to be thixotropic, the reduction or complete loss of the shearing action tends to adversely affect the ability of the drilling fluid to flush and carry away cuttings from the borehole. As a result, in deviated holes drilled with a downhole motor and bent sub alone, formation cuttings are more likely to settle out of the drilling fluid on the bottom or low side of the borehole. This may increase borehole drag, making weight-on-bit transmission to the bit even more difficult, and often resulting in tool phase control and prediction problems. These challenges encountered in sliding can result in an inefficient and time consuming operation.
Still further, drilling with the downhole motor and bent sub during a sliding operation deprives the driller of the use of a significant source of rotational energy and power, namely the surface equipment that is otherwise employed to rotate the drill string. In directional drilling cases employing a downhole motor powered by drilling fluid pressure (e.g., progressive cavity motor), the large pressure drop across the downhole motor consumes a significant portion of the energy of the drilling fluid, and may detrimentally reduce the hydraulic capabilities of the drilling fluid advanced to the bit face and borehole bottom. In other words, the large pressure drop across the motor results in a lower drilling fluid pressure at the bit face, potentially decreasing the ability of the drilling fluid to clean and cool the cutter elements on the bit face, and flush away cutting from the borehole bottom. To the contrary, when surface equipment is employed to rotate the drill string and the bit, rotational energy and power are directly translated to the bit, without the need to convert drilling fluid pressure to rotational energy. Consequently, the use of surface equipment to rotate a drill string and bit may result in increased ROP and improved bit hydraulics as compared to a bit rotated by a downhole motor alone.
In addition to deviating from vertical in directional drilling operations as shown in FIG. 1, it may also be desirable to have a drill bit capable of returning to a vertical drilling orientation in the event the drill bit inadvertently deviates from vertical. The ability of a bit to return to a vertical path after deviating from such a path is generally referred to as “dropping”. In order to effect dropping, a drill bit must have the capability of drilling or penetrating the earth in a direction not parallel with the longitudinal axis of the bit.
As shown in the schematic view of FIG. 2, a drillstring assembly 50 including a drill string 53 and a bit 51 is shown drilling a borehole 55 that has deviated from vertical. Drillstring assembly 50 has a weight vector 52 that consists of an axial component 54 and a radial or normal component 56. Unlike the directional drilling operations described above in which deviations from vertical are desired, in some cases, deviations from vertical are unintentional or inadvertent. In such cases, it may be desirable to return drilling assembly 50 to a vertical orientation while drilling. To effect such a return to vertical, drill bit 51 must drill in a direction that is not parallel to axial vector 54. This may be accomplished by cutting and removing formation material from a sidewall 57 of borehole 55.
Accordingly, there remains a need in the art for an apparatus or system capable of altering the azimuth or inclination of a drill bit and well without relying solely on a downhole motor or rotary steerable device. Such an apparatus would be particularly well received if it was capable of altering the direction of the drill string and borehole trajectory in a controlled manner while maintaining the rotation of the entire drill string. In addition, it is desired that this change in direction be achieved with a drill bit having predetermined dropping tendencies, regardless of formation type, lithology, well trajectory, stratigraphy, or formation dip angles.